I sadly think there are deep rooted prejudices in all societies and cultures. Our own understanding of this, and how we encounter it is very much based on our lived experience.
Being Scottish, but having lived in Portugal, Spain, Argentina and Brasil I have seen how they all have deep-rooted problems, which is why some of the very generalised statements on this thread about Argentina annoy me so much. I can't objectively tell you which of the countries above is the best - I have lived experience in all of them - some of those experiences were better than others - but just like every other place, they do all have their problems.
Before leaving Scotland, I had no lived experience of being treated differently for my nationality or skin colour, but I was very aware of sectarianism and coming from a catholic family, had experience of its impact. I should mention my dad is English, so I regularly visited England.
In Portugal, I had my first experience living somewhere where I was a foreigner, and that does inevitably change the way you experience things and how people perceive you - especially when you have to start dealing with the day-to-day realities - bank accounts, residency procedures, finding somewhere to live etc etc. While I didn't have any issues directly, I knew a lot of Brasilians and they often experienced a lot of racism - for example, if I phoned up to enquire about a rental property, I would always get a viewing, but there were occasions where a Brasilian friend would phone up and the landlord would claim the apartment wasn't available - and in fact, on a couple of occasions even admitted they did not rent to Brasilians.
As I have explained, so won't repeat, I experienced a lot of difficulty in Spain, particularly because of what my husband had to deal with. Another example would be the way people from Morocco are treated in Spain. But again, before relocating there, I had no idea of these issues as I had no lived experience of it.
Race is such a complicated topic in Brasil that I can't even begin to explain it, but race is so deep-rooted in the way Brasil is today and is still a strong source of injustice. While on the surface they have strong laws against racism, the problem is deeply structural and my day-to-day experience in Brasil as a white person is completely different to that of my husband. I found it difficult to live in Brasil long-term because of this - there were other factors too, but it was one of them.
Argentina, as I have said, also has problems with racism. Again, completely wrong and it should never be tolerated. A lot of it is to do with ignorance (the average person in Argentina has barely interacted face-to-face with someone from outside of the country), some cultural behaviours are different. In general, I suspect I do get treated better abroad for being Scottish rather than English - I don't think that's right - but it'd be unfair to deny it. Most people in Argentina think Scotland is independent (lots of Brasilians too), and those who know it isn't can't understand why not. That said, if you are English, act like a reasonable person (like the vast majority do) and avoid talking about Las Malvinas, you'll find people from here to be very nice and welcoming people who aren't actually very different to you. Ironically, I think it's the most English of all the places I've lived, but out of context, that'll make no sense.
Of course all societies and nations have deep-rooted problems in addition to those mentioned above e.g. poverty, class, wealth distribution, housing, etc etc. I won't go into my experiences of those individual issues. I have probably been boring enough with this post!
But to go back to the topic: at the end of the day, it's just a football match. Passionately support the team you want like a normal, decent person, and then when it's all over, after a moment to celebrate or commiserate, you move on. I can't get upset about someone supporting an opposing team. What does upset me is when people justify this based on sweeping generalisations, or comments rooted in ignorance, rather than fact or experience.